The invention relates to a method for storing a digital data base in a mass memory whose capacity is divided into a number of storage parcels each having a predetermined storage capacity, said method comprises the following steps:
dividing the data base into a number of main cells according to a predetermined regular division pattern;
dividing each main cell into a further number of data parcels whereby each data parcel covers a respective part of the main cell, which part has a data content occupying substantially completely the storage capacity of a storage parcel;
setting up an identifier table comprising:
(a) a first identifier identifying said predetermined pattern;
(b) a second identifier identifying said division into data parcels;
(c) for each main cell a data parcel list which comprises for each data parcel of that main cell an addresspointer indicating at which memory location the respective data parcel is stored;
loading into said mass memory the data content of each data parcel at the address indicated by its address pointer and loading said identifier table.
Such a method is known from the article "A file organization for Geographic information systems based on Spatial Proximity" of T. Matsuyama, Le Viet Hao, and M. Nagus and published in Computer Vision, Graphics and Image Processing June 26, 1984, No. 3 (New York, USA) page 303-318. In the known method the digital data base, which is formed by geographic information, is first divided into a number of main cells according to a regular division pattern, such as for example a quad tree Using that division pattern the digital data base is partitioned into four rectangular regions. The available storage capacity of the mass memory is divided into a number of storage parcels each having a predetermined storage capacity. In order to use efficiently the available storage capacity, the method checks for each main cell whether the data content thereof fits into one storage parcel. If this is the case, the data content of that main cell forms a data parcel and is stored into a storage parcel. However if the data content of a main cell exceeds the storage capacity of a storage parcel, that main cell is further divided into a further number of data parcels, such that each data parcel occupies substantially completely the storage capacity of a storage parcel. By that division into data parcels the data content is taken into account and so much data is grouped together until a storage parcel is substantially completely occupied. In order to access the stored data an identifier table is set up, which comprises a first identifier identifying the division into main cells, and a second identifier identifying the division into parcels. Further a parcel list is set up in order to indicate at which memory location the different data parcels are stored in the mass memory. After accomplishing that division of the data base, the identifier table and the different data parcel are stored at respective addresses in the mass memory.
A disadvantage of the known method is that the division of the main cells into data parcels is realized according to an arbitrary division pattern wherein only the data content is taken into account. The efficient use of the available storage capacity thus obtained is overridden by the need of storing a complicated second identifier which identifies the arbitrary division pattern used for the parcel division. Such a complicated second identifier not only requires much storage capacity but also renders the search of a particular parcel rather difficult and complicated.